In recent years, compact cameras and digital video cameras having high portability which are easily carried to anywhere and used at all times have been abundantly available in the market. As a result, there has been a demand for making the compact cameras and video camera which are lighter in weight, smaller in size and higher in performance. As a result of weight reduction, miniaturization and performance enhancement of optical instruments or, particularly, compact camera and digital video camera as such, there have been also a demand for weight reduction, miniaturization and performance enhancement for the constituting parts thereof.
Particularly in shutter blades and aperture blades used for optical instruments, there is a necessity for weight reduction as a result of the fact that shutter speed is becoming higher. Further, since it is necessary to shield the light by covering the front side of a picture-taking element such as CCD, light-shielding property is fundamentally necessary. Furthermore, reflectivity of the surface is desired to be low for preventing the leakage of light from the gaps among the blades. Still further, since temperature of inner side of the camera becomes high depending upon the actual using environment, heat resistance and dimensional stability are also demanded for shutter blades and aperture blades.
Up to now, thin plate of metal (such as SUS or Al) has been commonly used as a light-shielding film as such. In the case of a lens shutter for camera for example, metal plates are rubbed each other upon opening and closing the blade material and generate big noise when a light-shielding film of thin metal plate is used as the shutter blades and aperture blades. Similar noise is also generated in a projector. For reducing the noise, the blade is to be operated in low speed. In such a case, there is a problem that adjustment of light quantity cannot follow the changes in the image whereby the image becomes instable.
In view of the above problem and also in view of the weight reduction, use of a film based on synthetic resins as a substrate instead of thin metal plate is becoming the main stream. However, when an insulating synthetic resin film is used, there is a problem of sticking of dust due to the charge of static electricity whereby the light-shielding film is further demanded to have electric conductivity. In view of the above, the characteristics necessary for light-shielding film are said to be high light-shielding property, heat resistance, dimensional stability, low glossiness, electric conductivity, etc. In order to fulfill the characteristics of the light-shielding film as such, there have been proposals of using various materials and film structures.
For example, Patent Document 1 discloses a method for manufacturing a light-shielding film where a film in which carbon black is compounded with polyester film is sandblasted so as to make the film surface coarse and then an electrically conducting agent is applied thereto. Patent Document 2 proposes a light-shielding film where a light-shielding layer constituted from a binder resin, black fine powder having average particle size of not more than 1 μm, an organic filler having average particle size of 0.5 to 10 μm and a lubricant having average particle size of 0.1 to 10 μm is formed at least on one side of a substrate film. Patent Document 3 proposes a light-shielding film where a light-shielding layer constituted from thermosetting resin having Tg of not lower than 40° C. and having softening point of not lower than 80° C., black fine powder having average particle size of not more than 1 μm, an organic filler having average particle size of 0.5 to 10 μm and a lubricant having average particle size of 0.1 to 10 μm is formed at least on one side of a substrate film. Further, Patent Document 4 proposes a flexible printed circuit board having light-shielding property which is manufactured by application of a light-shielding ink comprising fine particles of silica having average particle size of 2 to 8 μm and aromatic polyamide-imide to a flexible printed circuit board.